Large mining machines accomplish the task of digging through a bank by suspending a bucket or dipper on steel hoist ropes that pass over steel sheaves mounted at the tip of a stationary boom. The ropes then connect to and wrap around a hoist drum mounted to the revolving frame structure. The hoist drum is motor driven through a gearbox attached to the revolving frame. Rotation of the drum in one direction hoists the dipper through the bank. Rotation of the drum in the opposite direction allows the dipper to be lowered.
During both of these operations, but specifically while under heavy load during the digging/hoisting operation, the rope span between the hoist drum and boom point sheaves can move up or down or side to side, i.e., vibrate significantly due to pulsations in axial load. Further, electric mining shovels frequently encounter situations that result in the dipper impacting something that causes stress in the hoist rope. The span can vibrate as much as six to twelve inches above and below the straight line of sight path from drum to sheave. This vibration induces bending stresses in the portion of the rope that is leaving contact with the drum. Because the vibration occurs during the dig portion of the cycle, the ropes are under the highest axial load that they will see. This forces the magnitude of the bending stresses due to the rope vibration to be high as well. These bending stresses are concentrated in the outer main strands of the rope. The repeated effect of these bending stresses due to the vibration of the rope span can fatigue the rope and eventually result in broken wires in the outside layers of the rope.
This bending fatigue in the wire rope results from rubbing between the wires in the rope main strands and the outer wires of the center strand. This rubbing eventually causes the wires to break. If a significant amount of the wires break, there may not be enough strength remaining to support the applied load. This can result in rope failure. Because many of the wire breaks may occur inside the wire rope and are unseen and difficult to detect by other means, the failure can come as a surprise.
Electric mining shovel hoist ropes can be taken out of service for many reasons. Some of these may be because the ropes are damaged due to impact or abrasion near the dipper; there is reduced rope diameter due to wear or loss of core support; there is corrosion near the end supports of the rope; or there are a number of broken wires in the outer layers of the rope. This last criterion is a very common reason for electric mining shovel hoist ropes to be removed from service. The typical location of the broken wires occurs in the portion of the rope length that leaves contact with the hoist drum during the operating cycle of the shovel. This is the failure mechanism that this invention is trying to combat.
One typical approach to increase hoist rope life is to increase the rope diameter in order to increase the overall strength of the rope. This increase in diameter can help to lengthen rope life, but the increase in diameter has limitations. Small increases in diameter, ⅛ inch (0.3 cm) or less, can sometimes be accommodated with the existing drum and sheave grooving, but this small increase alone will likely not have a profound effect on rope life. Larger increases require larger drum grooving and possibly increased groove pitch spacing. This increased pitch spacing can then lead to the necessity of a longer or larger diameter hoist drum.
Some current shovels include a mechanism in order to prevent the rope span from colliding with and damaging the boom handrails or other items attached to the boom. The mechanism is a steel frame, like a picture frame, attached to the boom, through which the hoist ropes pass. In another similar structure, the steel frame has adjustable upper and lower guides that constrain the movement of the hoist ropes, both the upper and lower guides have nylon rollers thereon. In some instances, the lower guide has been positioned close to the hoist ropes, but no attempt is made to reduce vibration by positioning the guide in constant contact with the hoist rope.